1. Technical Field
The invention relates to an amplifier circuit and an operation method thereof. Particularly, the invention relates to an amplifier circuit capable of changing a gain value and an operation method thereof.
2. Related Art
In a communication system, the amplifier circuit is used for converting voltage levels of signals and isolating signals at an input terminal and an output signal thereof. The amplifier circuit is generally composed of at least one stage of amplifier, and a first stage of amplifier of the amplifier circuit is generally a low noise amplifier. The greater the gain of the first stage of amplifier is, regarding noise of a whole system, the smaller the noise component contributed by the post stage of amplifier is. Regarding a high-frequency amplifier, an iterative method is generally used to increase isolation between the input terminal and the output terminal. When the single stage of amplifier cannot reach a higher gain, a multi-stage cascade method can be used to reach the higher gain, though regarding an input signal with higher intensity, due to the influence of the multi-stage cascade, a whole linearity of the amplifier circuit is worse than that of the single stage of amplifier, which influences an operation of a post stage circuit.